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Current computed tomography (CT) imaging systems may provide an annular gantry that receives a patient within a gantry bore and rotates about the patient.
The gantry supports an x-ray source to project, for example, a fan shaped x-ray beam extending along the plane of rotation of the gantry toward the bore. The x-ray beam will thus pass through the patient where it is then received by a detector array. The detector array is held on the gantry opposite to the x-ray source with respect to the bore.
As the gantry rotates, a series of x-ray projections of a xe2x80x9cslicexe2x80x9d of the patient are obtained at different angles. These projections are reconstructed mathematically, for example, using the well known filtered back projection algorithm, to create a tomographic image of that slice. The patient may be moved axially through the bore to obtain data on adjacent slices which may be assembled to provide data about arbitrary volumes of interest within the patient.
The rotational speed of the gantry affects the time necessary to obtain the tomographic image and thus, generally, higher speeds of rotation of the gantry are desired. Higher speeds increase the importance of static and dynamic balance of the gantry.
Current approaches to balancing the gantry attempt to control the center of gravity and mass of the components mounted on the gantry, to a tight specification, so that the assembled system is within balance. These components generally include the x-ray source and detector, signal processing circuitry, power supplies and cooling systems. The gantry may then be manually balanced by the addition of weights or movement of components, a time consuming and difficult task.
The need to precisely control of the center of gravity and mass of the components on the gantry increases the cost of these components. Tight specification of center of gravity and mass hamper design improvements and make multiple sourcing of the components more difficult. When a component is replaced in the field, the gantry may need to be rebalanced. Such field rebalancing is more difficult than balancing during manufacturing when the greater accessibility to the gantry, balancing weights, and balancing tools may be had.
The present invention attaches at least one electronically positionable weight to the gantry during the manufacturing process or in a retrofit operation. Movement of the weight corrects for imbalance and thereby allows much reduced tolerances for the mass and center of gravity of the gantry components. The weight may be optimally positioned on the gantry without concern for accessibility because it is electronically controlled. The electronic control further allows for the implementation of automatic balancing mechanisms that may be easily performed in the factory or in the field.
A key to the invention is the recognition that a limited set of such electronically positionable weights may provide for arbitrary static and dynamic gantry balancing, however, subsets of this ideal set of weights may also be used to advantage.